1. Field of the Invention
The present invention relates to a hysteresis switch and an electricity charging module using the same, and more particularly, to a hysteresis switch capable of supplying a stable charging power to a rechargeable battery using a simple circuit configuration, and to an electricity charging module using such a hysteresis switch.
2. Description of the Related Art
It is well known that WSN is a wireless sensor network that collects information from various sensors via wireless communication; it is comprised of sensor nodes and sink nodes, wherein the sink node collects information from the sensor nodes and transmits this collected information to the outside. Not like a conventional network, the main purpose of such a wireless sensor network is to provide an automated remote information collection means instead of communication means; as network technologies including wireless personal area network (WPAN) and Ad-hoc network have been developing rapidly, wireless sensor networks are being widely utilized in various applications including scientific, medical, military, and commercial application.
Types of sensors applicable to such wireless sensor networks are temperature sensor, acceleration sensor, position sensor, pressure sensor, fingerprint sensor, gas sensor, etc. Recently, a technology for acquiring logistics information flow by attaching a tag to an object based on radio frequency identification (RFID) technology has been introduced.
Since each sensor node uses independent power supply, i.e. battery, furthermore, rechargeable battery is widely used to extend the battery replacement period or in order not to replace the battery; therefore, energy harvesting technology or energy scavenging technology for charging the rechargeable battery by utilizing the energy emitted from the external energy sources is getting more attention. An external energy source suitable for this purpose can be a sunlight, an indoor illumination light source, RF energy, heat/vibration/kinetic energy of the sensing object or vicinity thereof; more than one external energy source among the above-mentioned external energy sources can be utilized, for example, a sunlight and vibration energy or RF energy. In case of sunlight, the power is supplied as a D.C. current after energy conversion; but in case of vibration energy or RF energy, the energy is converted to a D.C. power followed by rectification before it is supplied as a power.
Since the conventional battery charging module for charging rechargeable battery applies external power directly to the charging unit without using the storage capacitors, it may not sufficiently charge the rechargeable battery due to the lack of the charging current under energy harvesting environment.
Generally, for proper charging, the external charging voltage for a battery charging module should be about 10 to 20% greater than the nominal voltage of the rechargeable battery. Furthermore, if the supply voltage of an external power source fluctuates around the turn-on voltage level due to the noises or the environmental conditions, the switches will frequently be turned on and off resulting in unstable supply of the charging voltage for the battery, thereby degrading the charging efficiency and shortening the battery life.